An ignition apparatus for an internal combustion engine has been known that includes an ignition coil having a primary coil and a secondary coil, and an ignition plug connected to the secondary coil. In such an ignition apparatus, an arc discharge is produced in an air-fuel mixture using energy supplied between electrodes of the ignition plug by electromagnetic induction resulting from energization and de-energization of the primary coil.
A known ignition apparatus includes a plasma device that produces a plasma discharge in an air-fuel mixture, and causes the air-fuel mixture to contain radicals by producing the plasma discharge, to improve the ignition performance of the air-fuel mixture (see, for example, PTL 1 and 2).
In recent years, for improved fuel efficiency, the combustion of a lean air-fuel mixture (having an air-fuel ratio higher than the stoichiometric air-fuel ratio) in internal combustion engines has been considered. However, with the lean air-fuel mixture, the amount of energy to be supplied between the electrodes of the ignition plug needs to be increased to reliably burn the fuel, which makes the electrodes of the ignition plug susceptible to wear.
With the configurations disclosed in PTL 1 and 2, the ignition performance is expected to improve as a result of containing the radicals. However, the kind, amount, and the like of radicals generated by the plasma discharge depends on the physical condition (temperature, pressure, etc.). Therefore, improvements to the ignition performance are not always expected for the lean air-fuel mixture. Thus, for the lean air-fuel mixture, the voltage to be applied to the electrodes of the ignition plug needs to be increased as a solution. Accordingly, the ignition apparatus including the plasma device also requires some measures to deal with the wear of the electrodes of the ignition plug. Note that in the ignition apparatus of Patent Literature 2, the ignition plug also serves as the plasma device, and thus the electrodes are significantly subject to wear.